SANTA CLARA, Calif. (AP) — Patrick Willis believes the San Francisco 49ers can still be a great defense even without All-Pro tackle Justin Smith.

The 49ers might have to spend the rest of this season finding out whether or not Willis, their All-Pro linebacker and defensive leader, is correct.

Smith is unlikely to play Sunday in the regular-season finale against Arizona because of a partially torn left triceps. The injury could force Smith to miss some or all of the postseason.

The San Francisco defense has been gouged for big yardage and points since Smith was injured early in the third quarter Dec. 16 at New England. But the NFL's second-ranked defense hopes to regroup and regain momentum for the playoffs against an Arizona team that ranks last in the league in total offense.

Smith, one of five San Francisco defenders selected to start for the NFC in the Pro Bowl, is expected to be on the sideline against the Cardinals.

"You can't really put into words what he means to this defense," Willis said Thursday. "We most certainly miss him and hopefully we'll get him back for the playoffs.

"But we have a great defense and I truly believe that. When you have what we have on our defense, we should be able to go out there and display that each time. Starting from this Sunday, we have to make sure we go out and do that on a consistent basis and play like the defense we know we're capable of playing like."

The 49ers haven't been the same on defense since losing Smith in the victory at New England.

San Francisco surrendered four consecutive touchdown drives after Smith left that game with the 49ers leading 31-3. Without Smith over most of the past six quarters, the 49ers have allowed 753 yards, 66 points and 48 first downs.

Much of that damage came last week in a 42-13 loss at Seattle, the worst defeat of coach Jim Harbaugh's two seasons with the 49ers. Without Smith, a first-team All-Pro and Pro Bowl starter last season, San Francisco's defense allowed season highs in both points scored and rushing yards allowed.

"He's a great player, one of the best defensive players in the NFL, and anytime you lose one of those guys it does have an adverse effect on you," Fangio said. "But I didn't think the problems we've had, the last game in particular, were because of Justin not being there. We could have had five Justin Smiths. That wasn't the issue. We didn't play good, didn't coach good and those guys just whipped us."

Fangio has seen his defense bounce back before and expects it to do so again as the 49ers prepare for the playoffs. San Francisco needs a victory over Arizona to secure its second consecutive NFC West title, and the 49ers can capture the NFC's No. 2 playoff seed with a win over the Cardinals and a Green Bay loss Sunday at Minnesota.

The 49ers certainly have the defensive personnel to make do without Smith.

Willis, outside linebacker Aldon Smith and safeties Dashon Goldson and Donte Whitner all were named Pro Bowl starters on Wednesday along with Justin Smith. Middle linebacker NaVorro Bowman, San Francisco's leading tackler, also was selected to the NFC team, and cornerback Carlos Rogers and outside linebacker Ahmad Brooks are Pro Bowl alternates.

Willis, who became the first player in franchise history to be named to the Pro Bowl in each of his first six NFL seasons, doesn't expect San Francisco's defense to be down for long.

"It's like gravity," Willis said. "What comes up must come down. And what's down will eventually come up. For the most part, we have played well this season Now it's one of those things where each person has to take it up a whole another notch. We can't fill Justin's shoes - nobody can come in and do that - but I know we certainly can get it done. We're going to bounce back and make sure we do it right from this point on."

Veteran lineman Ricky Jean Francois made his first career start at right tackle in place of Smith last week and finished second on the team with six tackles.

Fangio's approach won't change whether it's Smith or Jean Francois playing that position the rest of the way.

"Justin's told me he's getting better each and every day and that's really all I know," Fangio said. "But it's like anything. You can lose anybody early in a game and you just have to go. We're too far along here to change the system because of one guy being out. We'll be fine."